In a compact dual gap solenoid design, packaging of the solenoid windings is difficult because of the requirement for cross-sectional area in the magnetic circuit. For a given package size and a given force level, the pole areas (inner and outer) determine the space remaining between the poles for windings and whatever structure is required to support them. This support structure usually takes the form of a molded bobbin, made of a non-conductive, and usually non-magnetic, material. Unfortunately, the material of the bobbin does not contribute to the cross-section of the magnetic circuit, and it displaces volume which would otherwise contribute to the number of turns of wire in the coil.
Previous attempts to make use of the space taken up by the bobbin, such as free-standing coils, metallic bobbins, and metal-filled plastic bobbins, have had limited success. This is because of the difficulty in meeting the requirement for very good insulation in order to maintain the electrical integrity of the coil, particularly when the terminals are molded into the bobbin.
From commonly assigned U.S. Pat. No. 5,065,128, it is known to injection-mold encapsulating material around an electromagnetic coil, thereby creating an encapsulated coil, and then to use the encapsulated coil in the solenoid of a solenoid-actuated valve, such as in U.S. Pat. Nos. 5,083,747 and 5,102,095, also commonly assigned.
From commonly assigned U.S. Pat. No. 5,226,221, it is known to support an electromagnetic coil on a holder that contains electrical terminals to which terminations of the coil have been attached, to injection-mold encapsulating material around the coil and holder except for distal end portions of the electrical terminals, thereby creating an encapsulated coil, and then to use the encapsulated coil in a solenoid.
Attempts to use plastic bobbins with ferrous material as filler have been largely unsuccessful because they contain a significant amount of conductivity which short circuits the coil. Using the bobbin as part of the magnetic circuit will only be successful if a solution is found for insulating the wires and terminals from the magnetic material.
It is seen then that there exists a need for an improved means of packaging the solenoid windings which overcomes the problems encountered in the prior art.